The present invention relates to pumps for liquids and in particular, although not exclusively, to pumps for steam condensate.
Pumps utilising chambers that may be allowed to fill by gravity to a chosen level and that are then pressurised using either the vapour of the liquid being pumped, or air (or sometimes an inert gas), to push the liquid from the chamber, are often described as “pressure-powered” pumps. The liquid enters and leaves the chamber through “non-return” or “check” valves. At the top of the chamber are two much smaller valves. The first one of these admits the pressurising or “motive” gas when it is open. The second one is a vent valve for releasing the motive gas from the chamber. The motive gas valve and the vent valve may be pneumatically actuated. In the case of one pattern presently used, the pneumatic signals to the valve actuators are controlled by electrical level probes in the chamber. Alternatively, the two valves may be actuated by electric motors or solenoids, these again responding to electric level probes or level switches.
Other pressure-powered pumps in use at present, such as the pump 1 illustrated in FIG. 1, have a relatively large float 2 carried on a lever arm 4 within the chamber 6. As the chamber fills with liquid, the buoyancy of the float 2 acting on the lever 4 applies force to one or more springs 8 which store energy as the float rises. At the upper tripping point the energy stored in the springs 8 is applied to a pushrod 10. This moves in such a manner as to close the vent valve 12 and to open the motive gas valve 14. The pressure in the chamber 6 then rises, closing the condensate inlet check valve 16, and at a sufficient value discharging the condensate through the outlet check valve 18.
As the condensate level in the chamber 6 falls, the float 2 is lowered, and its weight acting on the lever arm 4 again applies force to the springs 8. At the lower tripping point, the mechanism trips in the reverse manner and the energy stored in the springs 8 is applied to the push rod 10 in the opposite direction, so as to close the motive steam valve 14 and open the vent valve 12. The chamber pressure then falls as the motive steam is released and the next cycle begins. During the “discharge” phase of the cycle, condensate cannot enter the chamber, so a receiver is needed to accept and store the condensate until it can flow into the chamber at the start of the next cycle.
FIG. 2 shows an example of a pump 1 and an associated receiver 20 accepting condensate from a heat exchanger 22. Steam enters the heat exchanger 22 via a pipe 22A. The receiver 20 often is of a volume comparable to that of the chamber 6, and it is mounted at a height so as to permit gravity flow into the chamber at a desired rate. A trap 20A is fitted between the heat exchanger and the receiver 20. The drainage outlets 24 on the equipment from which the condensate is flowing must be at an even greater height to allow gravity drainage to the receiver 20 if the condensate is to flow when the source is at low or atmospheric pressure.
Condensate flow from the receiver 20 to the pump chamber 6 is intermittent, so the pipe sizes used often must be greater than those needed for continuous flow. Equally, flow in the delivery pipe 26 from the pump 1 occurs only during the discharge phase, so the instantaneous flow rate is higher than the average rate. Often increased pipe sizes are needed, compared with those that would be adequate with continuous flow.
Such existing pumps can be effective but have several drawbacks. First, they are inherently intermittent in action, requiring over-sizing of associated pipe work. Second, the pump chamber must be sufficiently tall to provide enough movement of the float, which also needs to be large itself, so that enough operating power is obtained to open and close the motive steam and venting valves against the pressures being used. Furthermore, the receiver must be at a sufficient height to allow gravity drainage to the pump chamber, and so steam-using equipment and steam traps often must be higher still. This can increase the costs involved in mounting the steam-using equipment at sufficient elevation or, where equipment is already installed, may preclude drainage to the pump of condensate.
Another disadvantage associated with existing pumps is that the operating power of the mechanism is stored in one or more springs that are highly stressed. The springs are compressed or extended and released twice during each cycle of the pump, and are subjected to the severe conditions that exist within the operating chamber of the pump. Any replacement of a failed spring can only be effected after removal of the mechanism from the pump chamber. Similarly, removal of the mechanism from the pump chamber is needed before any maintenance work needed on other parts of the mechanism can be performed, or re-adjustment of the settings of the tripping levels. If any electrically operated probes, controllers or motors are used then these require special protection in locations that are dirty, steamy, or where inflammable vapours may be present.